1. Field of the Invention
The present invention relates to a throttle valve control device for electrically controlling an opening of a throttle valve of an engine.
2. Description of Related Art
Explanation is made on a conventional throttle valve control device based on Japanese patent application laid-open No. 3-271528, which was filed by the applicant of the present invention. FIG. 8 shows a simplified view of the structure of the throttle valve control device disclosed in the above publication.
A throttle valve 110 is mounted on a shaft 103 of which an end is connected with a motor 102 for rotating the throttle valve 110. Regarding the opening/closing direction of the valve 110, a rotating direction (an opening direction) from a fully closed position to a fully opened position is regarded herein as the normal rotation direction, and a rotating direction (a closing direction) from the fully opened position to the fully closed position is regarded as the reverse rotation direction.
Another end of the shaft 103 is engaged with a lever 105 for fixedly holding the throttle valve 110 in a predetermined position when the engine is stopped. The lever 105 is connected with an end of a return spring 107, and another end of the spring 107 is fixed to a casing. The return spring 107 acts to always urge the throttle valve 110 in the reverse rotation direction through the lever 105. The lever 105 comes into contact with a stopper 108 at a predetermined position and is stopped there. The casing to which the return spring 107 is fixed is provided with a full close stopper 109 for holding the throttle valve 110 in the fully closed position. The lever 105 is also connected with an end of a spring 106, and another end of the spring 106 is connected with the shaft 103. The spring 106 acts to always urge the throttle valve 110 in the normal rotation direction.
The operation of the conventional throttle valve control device constructed as above is explained below. As shown in FIG. 9, the throttle valve 110 can be held in a fully opened position F, a fully closed position S, and a slightly opened position N.
The reason why the slightly opened position N is determined to be between the fully opened position F and the fully closed position S is to prevent the following possibility. In the case of the throttle valve 110 that is held in the fully closed position S whenever the engine is stopped, the valve 110 would be unable to open if the shaft 103 or the throttle valve 110 freezes under low temperatures in a cold district, etc., which results in an engine starting failure. The similar possibility may occur if adhesive materials such as combustion products and the like which generate between an air-intake tube and a valve with the engine motion adhere to the throttle valve 110. The slightly opened position N is therefore set so that the valve 110 is held in a state of opening by a predetermined amount from the fully closed position upon stop of the engine.
When the engine is stopped, the lever 105 is urged by the return spring 107 to a position where the lever 105 engages the stopper 108, while the shaft 103 is urged by the spring 106 to position where the shaft 103 engages the lever 105. Accordingly, the throttle valve 110 is held in the slightly opened position N.
In the operation of the valve 110 from the slightly opened position N to the fully opened position F, although the return spring 107 exerts the urging force on the shaft 103 in the reverse rotation direction through the lever 105, the driving torque of the motor 102 is so larger than the urging force of the return spring 107 that the throttle valve 110 is allowed to fully open. That is to say, driving of the motor 102 causes the shaft 103 connected with the motor to rotate and the lever 105 engaging with the shaft 103 to operate against the return spring force until the lever 105 comes into contact with the stopper 108.
In the operation of the valve 110 from the slightly opened position N to the fully closed position S, on the other hand, although the spring 106 exerts the urging force on the shaft 103 in the normal rotation direction, the driving torque of the motor 102 is so larger than the urging force of the spring 106 that the throttle valve 110 is allowed to fully close. That is to say, driving of the motor 102 causes the shaft 103 to rotate until the shaft 103 comes into contact with the full close stopper 109.
FIG. 10 shows a relationship between the opening of the throttle valve 110 and the urging force of the shaft 103, where the horizontal axis indicates the opening of the valve 110, S indicates the fully closed position thereof, N indicates the slightly opened position, and F indicates the fully closed position. The vertical axis indicates the urging force exerted on the shaft 103, taking the torque in the full-closing direction as a positive side and the torque in the full-opening direction as a negative side.
In the operation of the valve 110 from the slightly opened position N to the fully opened position F, the urging force of the return spring 107 at an initial value R.sub.0 is first exerted on the shaft 103 being in the slightly opened position N and then the urging force R gradually increases in proportion to the degree of the opening. In the operation from the slightly opened position N to the fully closed position S, the urging force of the spring 106 at an initial value P.sub.0 is first exerted on the shaft 103 being in the slightly opened position N and then the urging force P gradually increases as the opening decreases.
In this way, at the slightly opened position N, the direction of the spring torque on the shaft 103 is changed in reverse in correspondence to the urging force exerted on the shaft 103 by the spring 106 or 107. This needs the control to drive the motor 102 to reversely rotate at the slightly opened position N.
FIG. 11 shows a bridge circuit to control the motor 12. In FIG. 11, +B indicates a power source, and B1 to B4 indicate transistors. In the operation from the slightly opened position N to the fully opened position F, electric current from the power source is fed through the transistor B1, the motor 102, and the transistor B4 in order, whereby the motor 102 is driven to rotate in the normal rotation direction against the urging force of the return spring 107 acting in the reverse rotation direction. To the contrary, in the operation from the slightly opened position N to the fully closed position S, the current from the power source is fed through the transistor B3, the motor 102, and the transistor B2 in order, whereby the motor 102 is driven against the urging force of the spring 106 in the normal rotation direction.
According to the conventional device disclosed in the Japanese patent application laid-open No. 3-271528, though inconveniences caused by the freezing or the adhesive materials, etc. could be solved, the reverse rotation of the motor just at the slightly opened position N must be controlled in respect of both of the valve opening and closing directions. For such the control, an electronic circuit such as the bridge circuit shown in FIG. 11 is required. The conventional throttle valve control device must further have the two urging devices for urging the shaft in opposite directions and the lever, stopper, etc. which engage with the urging devices. This results in a complicated structure for a mechanism.
Consequently, there are problems of causing an enlarged throttle valve control device and an increase of cost resulting therefrom.